Method for blow molding and transfer molding thermoplastic material

ABSTRACT

A method of combining a transfer molding operation with a blow molding operation to form an article having wall sections of different thicknesses. The major portion of an extruded parison fed between a pair of mold sections and enclosed thereby is expanded into conformity with the cavity defined by the mold sections. An intermediate portion of the parison is directed into a cavity and displaced therefrom by opposed portions on the mold sections into a desired space to completely fill such spaced forming a wall section having a different wall thickness than the remainder of the article.

This is a continuation of application Ser. No. 520,622 filed Nov. 4,1974, now abandoned, the latter being a division of U.S. Ser. No.310,896 filed Nov. 30, 1972 and issued as U.S. Pat. No. 3,869,239.

BACKGROUND OF THE INVENTION

The present invention relates generally to the blow molding ofthermoplastic material and, more particularly, to a method of blowmolding plastic articles in combination with a transfer moldingtechnique. While not limited thereto, this invention is particularlyconcerned with the molding of plastic articles of one-piece constructionhaving at least one wall section of a different thickness than the wallthickness of the major portion of the finished article.

It has been found that blow molding is a relatively inexpensive andefficient process for molding hollow plastic articles or articles of adouble wall construction having substantially uniform wall thicknesses.However, a problem arises where it is desired to vary the wall thicknessof certain portions of the article being formed, especially where a wallsection of a thickness greater than the wall thickness of the parison isdesired. Also, the flowing thermoplastic material in a blow moldingoperation tends to thin out as the material is stretched during blowing,this thinning out being especially pronounced at relatively sharpcorners producing weaker portions at and adjacent to such corners.Moreover, voids or spaces are formed along the mold surfaces adjacentthese corners to produce irregularities in the finished surfaces. Thisis especially undesirable where such surfaces are intended as sealingsurfaces.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to overcome theabove noted disadvantages by providing a novel method of molding plasticarticles of one-piece construction having wall sections of varyingthicknesses and smooth, uniform outer surfaces.

It is another object of this invention to provide a method of blowmolding plastic articles in combination with a transfer moldingtechnique.

It is a further object of the present invention to provide a combinedblow molding and transfer molding method for molding plastic articleshaving wall sections of varied thicknesses.

The foregoing and other objects, advantages, and characterizing featuresof the present invention will become clearly apparent from the ensuingdetailed description thereof, taken together with the accompanyingdrawings wherein like reference characters denote like parts throughoutthe various views.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front elevational view of an illustrative plastic articleformed by the method and apparatus of this invention, showing portionsthereof broken away to illustrate the mounting thereof on a secondcomponent;

FIG. 2 is a fragmentary, vertical sectional view taken through thecentral portion of a mold assembly of this invention, showing the moldparts moving toward each other;

FIG. 3 is a view similar to FIG. 2 but showing the mold parts positionedcloser to each other;

FIG. 4 is a vertical sectional view showing the mold sections completelyclosed; and

FIGS. 5-7 are schematic views illustrating the mold sections of thisinvention in various positions relative to the tubular parison.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now in detail to the illustrative embodiment depicted in theaccompanying drawings, there is shown apparatus of this invention formolding a thermoplastic article of the type illustrated in FIG. 1. Thespecific article depicted in FIGS. 1 and 4 is a toilet bowl, generallydesignated 10, used in conjunction with a discharge receiving tank 11and comprising a relatively deep body of hollow, double wallconstruction formed by an outer shell having a side wall 12 and a bottomwall 14, and an inner shell 16 joined at its opposite ends to bottomwall 14 and side wall 12 of the outer shell at a juncture 18 and by atop wall in the form of a ledge 20 adapted to support a toilet seat (notshown).

Bowl 10 is provided with an outlet 22 (FIG. 1) defined by an annularwall portion 24 extending downwardly from the annular juncture 18between the inner and outer shells. As shown in FIG. 1, annular wall 24is telescopically received within the filler neck 26 of receiving tank11 upon which bowl 10 is supported. An annular seal ring 30 is disposedabout annular wall 24 for providing a fluid tight seal between wall 24and filler neck 26. In order to effect a proper fluid tight seal aboutan annular wall 24, it is important that the outer peripheral surface 32thereof be in a true round and smooth condition without any surfaceirregularities therein. It is also necessary to form the cantileveredwall 24 thicker than the wall thickness of the inner and outer shellsfor strength and added rigidity.

In accordance with the present invention, annular wall 24 is moldedintegrally with bowl 10 by a combined blow molding and transfer moldingoperation as follows:

With reference to FIGS. 5-7, a molten plastic material, such asplasticized polyethylene for example, is fed into an extrusion die 34 byconventional feed means, not shown, from a suitable supply 36 (FIG. 7).The molten plastic is extruded from die 34 as a tubular parison 38, andis gravity fed downwardly between mold sections 40 and 42 which aremounted for movement into and out of engagement about parison 38. Moldsections 40 and 42 can be pivotally mounted or otherwise supported andcan be moved by conventional drive means 44. The free end of parison 38is disposed below the lower ends of mold sections 40 and 42 before suchsections engage parison 38 to insure that the enclosed portion ofparison 38 extends from the top to the bottom of said mold sections 40and 42.

Mold sections 40 and 42 are provided with cavities 46 and 48,respecitvely, and when these mold sections 40 and 42 are engaged, asshown in FIGS. 4 and 7, cavities 46 and 48 form a composite mold cavitydefining the outer surface configuration of the finished bowl 10.

Mold sections 40 and 42 are provided with knife-edge formations 50 and52 (FIG. 4) extending completely around mold cavities 46 and 48 and areadapted to engage each other for severing and enclosing a portion ofparison 38 within the mold sections 40 and 42. If desired, either moldsection can be provided with a flat surface, serving as a bed plateengageable by the knife-edge formation of the other mold section.

Mold section 40 carries means for introducing expansion fluid into theenclosed parison portion, such means comprising a needle 54 which ismounted in mold section 40 and projects inwardly of the cavity definingface thereof through the wall of the enclosed parison. A source ofcompressed air 56 (FIG. 7) is connected via conduit 58 to needle 54 forintroducing compressed air into the enclosed, hollow parison, expandingthe same into engagement with the mold cavity walls, causing theenclosed parison to assume the shape of the composite mold cavity asshown in FIG. 4.

In forming the specially shaped body 10, it is important that theformation of annular wall 24 be of the desired thickness throughout andthat the outer surface thereof be uniform and smooth. It has been foundthat the flow of the thermoplastic material in a conventional blowmolding operation tends to thin out, especially about relatively sharpbends or corners, such as that provided at juncture 18 to preventcomplete filling of the annular space between the mold sections at suchcorners. As a result of the reduced wall thickness at such bend, thefinished annular wall portion has a weakened zone at the juncturethereof with the hollow plastic body rendering it especially vulnerableto shear forces acting at such juncture. Also, the flowing materialabout such bend does not completely fill the space adjacent thereto,producing minute voids or spaces adjacent the mold surface which resultin an irregular surface on the finished article. This, of course,presents problems, and is especially intolerable when such surface isintended as a sealing surface.

An important feature of the present invention resides in controlledtransfer molding means for displacing a portion of the thermoplasticmaterial during the blow molding cycle to desired areas for completelyfilling the same, insuring the formation of accurate and solid wallthicknesses and smooth surace conditions. To this end, an insert,generally designated 62, is releasably secured in mold section 40 by aplurality of fasteners 64. Insert 62 is provided with an axial borehaving a first portion 68 defined by a cylindrical wall surface 70, anintermediate bore portion 72 of reduced diameter, and an enlargeddiameter portion defined by a cylindrical wall surface 74 forming achamber 76 therein. A reciprocable back-up member, in the form of apiston 78 having an outer face or surface 79, is mounted in chamber 76and is provided with a rod 80 extending through bore portion 72. A capplate 82 is removably secured to the outer end of rod 80 by a fastener84 to facilitate removal of piston 78 from insert 60. Plate 82 serves asa stop engagable with an abutment shoulder 86 formed between boreportions 68 and 72 to limit extended movement of piston 78 toward theother mold section 42. Suitable O-rings 88, 90 and 91 are provided aboutthe peripheries of rod 80, piston 78 and insert 62 to prevent fluidleakage thereby. Chamber 76 is connected to a suitable source of fluidpressure, such as the compressed air source 93 (FIG. 7) via conduit 92and a control valve 94 for biasing piston 78 into its extended positiontoward mold section 42.

The inner end of insert 62 is provided with a portion 96 of reduced wallthickness having an inner surface 98 defining a further enlargeddiameter bore or opening 100 (FIG. 2) communicating with chamber 76 andbeing concentric therewith. Opening 100 terminates in an abutmentshoulder 102 which serves as a seat for the end portion of a maleprojection forming a part of mold section 42, as will presently appear.Surface 98 and abutment shoulder 102 define molding surfaces which formthe outer portion of annular wall 24 of the finished body 10. Surfaces74 and 98, abutment shoulder 102 and the upper face 79 of piston 78define a well or cavity 103 for receiving an intermediate portion of thethermoplastic material upon closing movement of mold sections 40 and 42.

The other mold section 42 is provided with a bowl-like portion 104facing mold section 40 and having a cylindrical male projection 106formed integral therewith and adapted to be received within opening 100of insert 62. Projection 106 has a tapered peripheral surface 108 whichserves as the opposite molding surface for the inner portion of annularwall 24 of the finished body 10. The axial end of projection 106 isprovided with a boss 110 of lesser diameter than projection 106 and isadapted to be received in chamber 76 when mold section 40 and 42 arefully closed. The transition between projection 106 and the reduceddiameter boss 110 defines an annular shoulder 112 adapted to seat onabutment shoulder 102 when mold sections 40 and 42 are closed, as shownin FIG. 5.

As mold sections 40 and 42 move toward a closed position about parison38, boss 110 engages a portion of one wall 38' of parison 38 and urgesit against a portion of the other wall 38" thereof, carrying bothparison wall portions therewith into cavity 103, as shown in FIG. 2.Accordingly, a parison double wall thickness is directed into cavity 103between the opposed faces of boss 110 and piston 78.

Prior to the closing of mold sections 40 and 42, air under pressure isdirected into chamber 76 behind piston 78 to urge the latter into anextended postion, as limited by the engagement of plate 82 with abutmentsurface 86, as shown in FIG. 2. This provides a resiliently biasedback-up surface 79 against which the double wall parison portion isforced by boss 110. As boss 110 continues to move into cavity 103 towardsurface 79 during the closing movement of mold sections 40 and 42, thepressure differential caused by the greater pressure acting on pistonface 79 than the air pressure acting against the rear face of piston 78causes the latter to be moved rearwardly in chamber 76. When these twopressures equalize, piston 78 will remain stationary, as shown in FIG.3, and upon further relative movement of boss 110 toward piston 78during the final closing movement of mold sections 40 and 42, thethermoplastic material in cavity 103 is partially displaced therefromand extruded through the annular clearance between shoulders 102 and 112into the annular space defined between wall surface 98 and projectionperipheral surface 108. Thus, a portion of the thermoplastic materialdeposited in cavity 103 is positively displaced or transferred therefromand forced into the annular space between opposed wall surfaces 98 and108 to form annular wall 24 of the finished body 10. By transferring theplastic material under the influence of positive pressure displacementunder controlled conditions, this annular space is completely filled,assuring the desired wall thickness of the finished annular wall 24 aswell as the formation of smooth and true round surfaces thereon. In thespecific article shown in the illustrative embodiment, annular wall 24is made thicker than the wall thickness of bowl 10 and thicker than thewall thickness of parison 38. However, the annular wall 24 can be madethinner than the wall thickness of bowl 10, if desired, by means of thepositive transfer molding technique incorporated in the molding processand apparatus of this invention. As a result, wall sections are notlimited to uniform thicknesses as is common in blow molding processes,but can be varied, as desired, by incorporating the above describedtransfer molding operation in the blow molding cycle.

Upon the closing of mold sections 40 and 42, shoulder 112 engagesabutment shoulder 102 (FIG. 4) to sever the disc-shaped portion ofthermoplastic material remaining in cavity 103. However, shoulder 112can be formed so as not to abut against shoulder 102 when mold sections40 and 42 are closed, thereby leaving a small clearance therebetweenmaintaining the thermoplastic material intact in one piece. Thedisc-like portion of thermoplastic material closing the outer end ofannular wall 24 can be subsequently removed after the finished articleis removed from between mold sections 40 and 42.

After mold sections 40 and 42 are closed, air is introduced into theenclosed parison 38 via conduit 58 and needle 54 to expand the parison38 into engagement with the mold cavity walls, causing the remainder ofthe parison 38 to assume the shape of the composite mold cavity as shownin FIG. 4. Mold sections 40 and 42 are cooled, as by means of coolingwater ducts 118 connected to a suitable source of cooling water 120(FIG. 7). This cools and sets the molded plastic, causing it to retainthe mold configuration. After the molded article has set for a shortperiod of time, mold sections 40 and 42 are opened and the finishedarticle is removed therefrom.

The cavity walls of mold sections 40 and 42 can be suitably vented toprevent the entrapment of any air between parison 38 and such cavitywalls during the blow molding operation. The introduction of expansionfluid into parison 38 causes the same to expand into conformance withthe cavity wall defining surfaces of mold sections 40 and 42. Undesiredinward collapsing of the parison wall is avoided by the force of theexpansion fluid and the elimination of air between parison 38 and moldsections 40 and 42.

The feeding of the raw thermoplastic material into extrusion die 34, theparison extruding operation, the opening and closing of mold sections 40and 42, the cooling thereof, the supplying of air under pressure tochamber 76, and the introduction of expansion fluid into parison 38 allare controlled in a timed relationship by suitable programming orcontrol means 130. Since such controls are conventional, in and ofthemselves, they are only schematically shown in FIG. 7 and it isbelieved that no further description or amplification is necessary.

Although the thermoplastic material used in the above described moldingoperation is stated to be polyethylene, it should be understood that anysuitable thermoplastic material can be employed, as desired. Likewise,this invention is not restricted to a method and apparatus for formingthe specific bowl 10 depicted in FIG. 1, but has utility in forming anyconfigurated plastic articles in which it is desired to form wallsections of varied thicknesses. Also, it should be understood that anysuitable biased back-up member, such as a spring loaded plate forexample, can be used in lieu of the fluid pressure biased piston 78, ifdesired, within the purview of this invention.

From the foregoing, it is apparent that the objects of the presentinvention have been fully accomplished. An improved method and apparatusis provided for forming an article having wall sections of variedthicknesses by a combined transfer molding and blow molding technique.Moreover, the outer wall surface conditions of critical portions of thearticle can be effectively controlled by incorporating a transfermolding operation in a blow molding cycle.

A preferred embodiment of this invention having been disclosed indetail, it is to be understood that this has been done by way ofillustration only.

I claim:
 1. A method of molding a hollow body comprising: introducingthermoplastic material between a pair of mold sections, moving said moldsections into engagement about the thermoplastic material to sever andenclose a segment thereof, directing a portion of said enclosedthermoplastic material into a cavity formed between opposed portions ofsaid mold sections, further closing said mold sections and providing apredetermined pressure in said cavity to act on said portion ofthermoplastic material to thereby displace at least a part of saidthermoplastic material portion from said cavity into an adjacent spacedefined by other portions of said mold sections upon closing movementthereof to completely fill said space and form a wall section on saidbody, and expanding the remainder of said thermoplastic material againstsaid mold sections to form said hollow body.
 2. A method according toclaim 1 wherein said segment of enclosed thermoplastic material includesopposed wall portions and said portion of said enclosed thermoplasticmaterial comprising both correspondingly opposed wall portions of saidenclosed thermoplastic segment.
 3. A method according to claim 1 whereinsaid predetermined pressure acts on a piston means disposed in saidcavity so that the latter means operates to displace said part of saidthermoplastic material portion from said cavity into said adjacentspace.
 4. A method according to claim 1 wherein said remainder of saidthermoplastic material is expanded by introducing a fluid thereintoafter said mold sections are closed.